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Intake Flow Problems at Low-Head Hydropower

Petr Lichtneger

The questions of intake flow conditions mostly associated with low-head run-off-river plants are discussed in the paper. As an example an intake flow non-

uniformity and the arising problem with shaft vibration is presented to documentthe flow influence. Finally the IFLOW project is introduced which is going to dealwith intake flow phenomena and possibilities of their simulation and optimisation.

Im Beitrag wird die Problematik der Einlaufstfmung diskutiert, die am meistenmit Wasserkraftanlagen der niedligeren Fallhehen verbunden ist. Am Beispielwerden eine Anstr6mungsungleichmaBigkeit und die entstehenden Probleme mitWellenvibrationen priisentiert, unt die Wirkung der Str6mung zil dokumentieren.SchlieBlich wird das Projekt IFLOW vorgestellt, das sich mit der Einstramungs-problematik und den M6glichkeiten der Strdmungssimulation und Optimierungbeschitigen wird.

1 Introduction

An effective exploitation of renewable resources has been increasinglyaccentuated in the recent years. Operators of the existing water power plantstherefore carry out overhauls in order to increase the efficiency and output of a

machine. First of all, it is the modernization of the turbine hydraulic profile, i. e.

modernization of the runner. However, the desired efficiency and turbine outputmay be reduced by hydraulic losses in an incorrectly profiled intake part. Apartfrom these, there are other characteristics of impropriate hydraulic conditions:

- Formation of free surface vortex accompanied by air entering the turbine

water passage;

- Increase in vibration caused by additional radial forces, as a result of non-

uniform influx on the runner.

High-head water power plants usually have their intakes designed in accordancewith well-proven methods. However, with low-head run-off power plants(usually equipped with a Kaplan turbine) water intake structure geometry can

vary significantly from case to case.

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Intake Flow Problems atLow.Head Hydropower

Under optimal working conditions of a water machine the streamlines and the

guide vanes surfaces should be parallel; otherwise an additional local loss occurs

on the guide vanes. Non-uniform influx causes irregular load on the turbine

runner producing additional radial forces acting on the rotor. Influx swirlingstruchires also indicate an improper solution of the intake configuration design.Strong su/face vortices may also suck air into the turbine water passage, thus

adversely affecting the turbine efficiency and vibration.

2 A case ofirregular intake flow at run-off-river power plant

A particular case of irregularity of intake flow at a run-off water power plantwill be presented here. The ill-designed intake structure induces a non-uniform

loading of the turbine and its increased level in shaft vibration. The objectivehydro power plant Vrane (1936) with two vertical units with Kaplan turbines

and the generating capacity of 2x9 MW is situated on a right bank of Vltava

River in Czech. The next figure shows the HPP location on a satellite picture.The connection to the main river bed is illustrated there as well as the wake zone

at the weir pile which seems to be the essential problem of the flow irregularityas will be demonstrated below.

Figure 1 The HPP Viane Situation

After the Unit Nr. 2 (the left one at the weir pile) was refurbished in 2007 the

commissioning and field acceptance tests were done to verify the hydraulicmachine condition. Surprisingly a higher level of shaft vibration occurred at

turbine radial bearing. A causal study brought an explanation in the uneven flow

at entrance section of the pressurized part of the turbine water passage.

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2.1 Fiow measurement

During the turbine efficiency measurement the discharge was gauged with OTT

current meters. A horizontal frame with 12 current meters was slowly draggeddown and up across the cross-section in grooves of inspection gate behind the

fine rack. The flow measurement was done in accordance with the direct

integration method (refer to ISO 3354). Thus the perpendicular mean velocities

in 12 perpendiculars were measured for the down and up direction separately,which were then integrated across the width to determine the total flow rate.

Because the intake structure was split into two channels to achieve smaller

inspection gate area, all measurements were done twice, in the left and in the

right intake channel. The result charts showing the distribution of perpendicularmean velocity across the intake width presents the Fig. 2. The red shaded

regions and arrows document the influence of the running unit no. 1 on the unit

no. 2 when the flow distribution goes to the right slice yet more. When runningunit no. 2 only the right to left slice flow ratio equals to 1.25 for all discharges(from 25 to 94 m /s), when running both the unit no. 2 and no.1 the ratio

increases to 1.35 (2x40 m /s).

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262 Intake Flow Problems at Low-Head Hydropower

Figure 3 Hydrometry frame with 12 current meters above the right inspection gate slice

opening

To analyse the flow quality more precisely the multi-point measurement byvelocity-area method was carried out for discharge 67 m /s. The resultingvelocity fields (see Figure 4) in the left and right slice validate the intake flow

non-uniformity; the right slice shows a flat flow distribution whereas the left

slice shows significant regions with smaller velocity values which are supposedto be caused by a wake region in front of fine rack in the intake bay and /or a

partially blinding of the fine rack. The wake zone flow disturbances were clearlyobserved (see the wake zone in Fig. 1).

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Figure 4 Velocity fields in the pressurized intake section- the left and the right slice

separated with a pillar (the regions with maximal velocities are red coloured, the

regions with minimal velocities are blue coloured; all values are positive)

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2.2 Shaft displacement measurement

The shaft displacement was measured in turbine radial bearing plane and inlower and upper generator bearing planes. The measuring plane at turbine

bearing was set ca 1.3 m above the upper bearing brass edge actually. Themeasured maximal kinetic shaft displacement Smax (refer to ISO 7919) in

dependence on the discharge is shown in Fig. 5. It's evident that the shaftvibration above the turbine bearing rises up with the discharge compared to the

vibrations nearby the generator bearings.

Smax [mm]

0.250 1.,m e.., r,;, 19#1 , ,t '11  :  ' 1 1'

 . •Smax_1 (turbinebearing measuringplane) t-· -tt-t- --·i-1- -t--1-f- - -1-- -r7-T-F--T-F-1-T--9-T-F-1-1---t#-t-*--t-1--1-t--1-9-/0-

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Figure 5 Maximal kinetic shaft displacement Smax measured by contact less B&K

displacement transducers

2.3 Shaft flexure

The signal phase analysis was done to find out what exactly happens. It seems

the turbine-generator shaft flexes the more the discharge rises. The hypothesis of

uneven loaded runner by the non-uniform influx onto the runner was checked up

upon the static elastic shaft line. The turbo-generator cored shaft was loadedwith a moment of force couple of trial value 20 kNm at point of turbine runner

junction. The calculated flexure line matches the measured shaft displacementfor given trial shaft load qualitative very well; see the Fig. 6 and 7.

263

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Intake Flow Problems at Low-Head Hydropower

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Figure 6 Flexure line calculated for a kial load on the runner (note: the turbo-generator is

vertical although there is an horizontal drawing in the picture)

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Eigure 7 Flexure line from on-site measurement at discharge of 80 m3/s (note: the turbo-

generator is vertical although there is an horizontal drawing in the picture)

3 The IFLOW project

Regarding the general support of power renewable resources, the resolved

problems are very current nowadays. It is important to deal with a question, to

what extent the ill-shaped intake structures influence the turbine behaviour.

That's why a project titled: "Intake Flow Simulation and Optimisation for

Hydropower" was proposed with main goals as follows:

- To identify the negative inf[uence of ill-designed intake structures at run-

off river power plants on the water turbine efficiency and the rotor

vibrations;

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Wasserbaukolloquium 2009 Wasserkraft im Zeichen des Klimawandels

Dresdener Wasserbautiche Mitteilungen Heft 39

- To find and prove the methodology of intake structure geometryoptimization by means of the numerical flow simulation and the physicalmodelling, and to reach the best conditions for maximal turbine efficiencyand its output.

The project combines civil hydraulic engineering with mechanical hydraulicengineering. Turbine producers focus on the design of a runner and adjacentwater passage, whereas civil engineers concentrate on a broader area of free

surface. The project is going to solve the complex intake flow interdisciplinaryconcerning the entire intake section configuration.Within this project the entire intake configuration of a low-head run-off river

power plant is understood: the nearest weir basin portion, the intake bayconnection to the weir basin, the intake structure itself with emergency and

inspection gate sections, and the (semi-) spiral case including the distributor andturbine chamber.

To solve such problems the optimization methods are to be implemented leadingto geometrical adjustments of the intake part that for a design head and flow rateshall ensure:

- Uniform flow distribution in the area in front of the runner;- Streamlines parallel with the guide vanes;- Flow without swirling phenomena and wake zones.

Three dimensional CFD simulations have proved effective for the design ofwater turbine hydraulic profile. For the design of intake profile and its location,simplified 2D depth-averaged models are used, which however are not quitesuitable for computation of flow closest to the intake stmcture. Both approachesmeet at an imaginary boundary formed by an intake entrance or another givensection in water passage.

Variant flow computations enable to determine the influence of tie intake

geometry on the hydraulic conditions in front of the runner, or on the runner

itself. The optimal adjustment of the intake geometry will be sought and thefinal option will be verified on a miniaturized model of the objective water

power plant in the hydraulic laboratory. The modern advanced methods of laser

anemometry will be used for measurements at the physical model.

The project aims to examine the area of water intakes, to simulate  ow in a

wider range (including the intake bay connection to the weir basin), to analyzethe causes of undesired phenomena and to suggest optimization proceduresleading to its removal.

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Intake Flow Problems a[Low-Head Hydropower

The obtained results should give the answer to how significant it is to assess the

construction part of reconstructed water power stations and what negativeeffects may ill-designed intakes have on the turbine behaviour.

4 Conclusion

The paper primarily gives information about intake flow problems like the non-

uniformity influx to the turbine. No optimal working conditions of low-head

hydro power plants with inaccurate designed intake structure regarding the water

influx (non-)uniformity to the turbine runner as mentioned above were the

motivation for the IFLOW project proposal which is assumed to be started in

2009. The project with both the computational and the physical modelling partwill be implemented at the Dresden University of Technology, Institute of

Hydraulic Engineering and Applied Hydromechanics, and with support of

European Commission.

References

Standard IEC 60041: Field acceptance tests to determine the hydraulicperformance of hydraulic turbines, storage pumps and pump-turbines.

Standard IEC 62006: Hydraulic machines - Acceptance tests of small

hydroelectric installations.

Standard ISO 7919: Mechanical vibration - Evaluation of machine vibration

by measurements on rotating shafts.

Standard ISO 10816: Mechanical vibration - Evaluation of machine

vibration by measurements on non-rotating parts.

Cepa,Z. Lichtneger,P. Novotny,J.: Field Acceptance Measurement HPP

Vrane after Overhaul. Technical Report, Litostroj Power - CKD

Blansko Engineering, a.s., 01/2008.

Stegner,P.: The CBE Statement of the Turbine Shaft DisplacementMeasurement Results on the Unit No.2 HPP Vrane after Overhaul.

Internal Report, Litostroj Power - CKD Blansko Engineering, a.s,

12/2007.

Author:

Ing. Petr Lichtneger, Ph.D.

CKD Blansko Engineering, a.s.

Capkova 2357/5, CZ-678 01 Blansko

Tel.: 4420-533309550

GSM:+420-604546650

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